The present invention relates to a process and apparatus for at least partial liquefaction of a fluid or a gaseous mixture at least partially formed from a mixture of hydrocarbons, for example natural gas. Natural gas is currently produced at sites distant from where it is used and it is normally liquefied so that it can be transported over long distances, for example by a LNG tanker, or stored as a liquid. The term xe2x80x9cnatural gasxe2x80x9d as used in the present description means a mixture formed mainly from methane, but which can also contain other hydrocarbons and nitrogen, regardless of the state it is in (gas, liquid or two-phase). The starting natural gas is mainly in the gas state, and at a pressure such that during the liquefaction stage, it can be in different states, for example liquid and gaseous, co-existing at a given time.
Prior art processes that have been used and disclosed, in particular those in United States patents U.S. Pat. No. 3,735,600 and U. S. Pat. No. 3,433,026, describe liquefaction processes principally comprising a first step during which natural gas is pre-cooled by vaporising a refrigerant mixture, and a second step that carries out the final natural gas liquefaction step, and produce a liquefied gas in a form that can be transported or stored, the cooling during that second step also being produced by vaporising a refrigerant mixture.
In such processes, a mixture of fluids used as a refrigeration fluid in the external refrigeration cycle is vaporised, compressed, cooled by exchanging heat with an ambient medium such as water or air, condensed, expanded and recycled.
The refrigerant mixture used in the second step in which the second refrigeration step is carried out is cooled by heat exchange with the ambient cooling medium, water or air, then the first step in which the first refrigeration step is carried out.
At the end of the first step, the refrigerant mixture is in the form of a two-phase fluid comprising a vapour phase and a liquid phase. Said phases are separated, for example in a separator drum, and sent, for example, to a spiral-tube heat exchanger, in which the vapour phase is condensed, while the natural gas is liquefied under pressure, cooling being ensured by vaporising the liquid fraction of the refrigerant mixture. The liquid fraction obtained by condensing the vapour fraction is sub-cooled, expanded and vaporised to ensure final liquefaction of the natural gas, which is sub-cooled before being expanded through a valve or a turbine to produce the desired liquefied natural gas (LNG).
The presence of a vapour phase necessitates carrying out a condensation operation on the refrigerant mixture at the second step and requires relatively complex and expensive equipment.
U.S. Pat. No. 4,195,979 describes adding a natural gas expansion step between the two refrigeration steps.
The Applicant""s French patent FR-A-2 743 140 proposes using pressure and temperature conditions selected to produce a completely condensed single-phase refrigerant mixture at the outlet from the first refrigeration step.
This causes constraints, which can adversely affect the economics of the process; in particular, the pressure to which the refrigerant mixture used in the second step is compressed can be relatively high.
A further prior art disposition consists of operating using three refrigeration cycles in series, each being operated with a pure body as the refrigerant. A first cycle functions with propane and enables ethylene to be condensed under pressure at a temperature of about xe2x88x9235xc2x0 C. Vaporisation of the ethylene at a pressure close to atmospheric pressure in a second cycle can condense methane below a temperature of about xe2x88x92100xc2x0 C. Vaporising the methane can sub-cool the liquefied natural gas (LNG) produced and thus expands it to enable it to be stored and transported at a pressure close to atmospheric pressure. This modus operandum has the disadvantage of having to use substantially pure ethylene which then has to be vaporised to condense substantially pure methane which is then itself vaporised to sub-cool the LNG. The use of a substantially pure body deleteriously affects the process costs and the use of ethylene, a particularly reactive unsaturated compound, necessitates taking particular precautions that also deleteriously affect the process costs.
The present invention concerns a process for partial liquefaction of a fluid G or a gaseous mixture at least partially formed from hydrocarbons, such as a natural gas NG, and an apparatus for carrying out this process. The process of the present invention at least partially overcomes the disadvantages cited in the prior art.
More precisely, the present invention concerns a process for partial liquefaction of a fluid G at least partially formed from hydrocarbons simultaneously producing a fraction that is liquid after expansion and a gaseous fraction representing at least 10% by weight, preferably 20% by weight, more preferably at least 30% by weight with respect to the weight of fluid G initially introduced into said process, and comprises at least two refrigeration steps during which:
in the first step a), the essentially gaseous fluid G is cooled using an external refrigerant M such that at the end of said first step, it is at least partially liquid, preferably completely liquid at the operating pressure, preferably about 4 to about 7 MPa; and
in the second step b), liquefaction of said fluid G is completed if necessary and said fluid G is sub-cooled, using a portion of the same fluid G, said portion being thus expanded and vaporised to produce the cooling necessary to recover the other portion of said fluid G that is completely liquid.
In a first variation, at least a portion of the gaseous fraction, representing at least 20% by weight with respect to the weight of the fluid G initially introduced into said process, can be used to produce electricity.
In a second variation, at least a portion of the gaseous fraction, representing at least 20% by weight with respect to the weight of the fluid G initially introduced into said process, can be re-injected into the zone from which it is recovered, and particularly in the case where the fluid G is a natural gas, into the well from which it is recovered.
The first refrigeration step comprises a plurality of heat exchange zones, for example, and cooling can be carried out in said successive heat exchange zones using an external refrigerant M that is expanded and vaporised at decreasing pressures. In a particular implementation of the invention, fluid G leaves the first refrigeration step as a single condensed phase. In a further implementation of the invention, fluid G leaves the first refrigeration step as a dense phase.
The external refrigerant M comprises at least one hydrocarbon, preferably at least two hydrocarbons. The hydrocarbon or hydrocarbons are preferably selected from the group formed by methane, ethane, propane and butanes. In a particular implementation of the process of the invention, the external refrigerant M comprises methane, ethane, propane and at least one butane.
The second step comprises a single exchange zone, for example, in which the liquefied fluid G is sub-cooled. At the outlet from this exchange zone, the liquefied gas is separated into two portions: a portion being sent to storage after expansion, the other portion being expanded and returned to the same exchange zone to produce, by vaporisation, the cooling necessary for the sub-cooling and optionally, when fluid G entering said second step is not completely liquid, to produce total liquefaction of said fluid G. In a particular implementation, the portion of fluid G used to produce the cooling necessary for this second step is vaporised at different decreasing pressures.
A preferred option for the second step is as follows: the liquefied gas is expanded to an intermediate pressure in the range 0.3 to 1.2 MPa at the outlet from the second step using either a liquid turbine or a Joule-Thomson valve. Fluid G is completely liquid after this first expansion. Fluid G is then separated into two substantially equal portions: one portion is normally sent to cryogenic storage after expansion, optionally after a denitrogenation step comprising partial revaporisation, a portion of the remainder being returned at the intermediate pressure and the other portion at a lower pressure to step b) to produce the cooling necessary for sub-cooling, and optionally when fluid G entering said second step is not completely liquid, for total liquefaction of said fluid G.
The operating conditions for the process of the invention are preferably selected such that the quantity of liquefied gas obtained is about 20% to about 80% by weight, more preferably about 30% to about 70% by weight of the quantity of gas entering the process.